If you're anything like me, you're a procrastinator, and a bad one at that!  You're also a perfectionist, and are very good about making something look impressive and fun to look and play in, but it is a very, VERY big deal to have to start ANYTHING creative, because you know the time and commitment involved that has to go into such a project.  So, in hopes of finding further motivation to go on once you've started the project, you'll begin anyway.  You have it planned to a T, or at least have a really good idea about how the whole thing will pan out.  You've even set deadlines for yourself because you know how bad you are otherwise.
In the end though, you lose motivation.  The map gets stale to look at, and it's no longer fun to keep going.  You know what has to be done, but it's sooo monotonous, and too much work you want to commit to something that maybe no one will play! -or will simply take way too long to finish within the coming months.  Perhaps you hit a brick wall in editing, that you can't seem to find an answer for, and the work-around is too daunting to commit towards.  These are all unfortunate realities in the mapping community that amateur mappers face every time they sit to map, whether they have outside motivating forces (a mod team, critiques, playtests), or simply are appeased with just showing screenshots to get nuggets of feedback to go on.  Mapping is difficult to complete!
In order to be successful, you have to admit there are boring parts about the job.  It's not all daisies and roses through a field of awesomeness; you're gonna have to put up with a great deal of frustration and boredom to overcome your goals.  Don't place your expectations high in your design; accomplish what you can, and make goals you're unsure you can do as optional.  For example: When designing the SnarkPit Compilation Map, I made the train optional cause I wasn't sure if I could do it.  But the rest of the map came together just fine.  Even optimized it well, but the train is still missing, but that's ok for now.

A plan never fails.  Having a plan goes a LONG long way in helping you divvy up tasks and goals.  A calendar may not always be a good solution to drive a mapper, (unless you're employed), but at least a simple list of "To Dos" can help things move along.  In fact, this list would be imperative nearing the completion of a map.  While playtesting, make a running list of everything you see that needs to be fixed, completed, or added.  This helps focus your attention on the most important tasks.
Slow and steady wins the race....  It's ok to break ground fast, because you're excited to see this thing become a reality, but planning again can go a long way.  And planning something should never be rushed.  Go at it as fast as you'd like, but all the better if you have a plan on what to do.  Research, research, research.  Always look for precedents.  Play maps that have been successful before you, and take screenshots of them.  Start an image folder of level precedents and real world locations for inspiration.  Define what you want to do, and the rest will be derived from that.  You can't go wrong relying on previous success to establish your own.  You can only be just as good, or better!
In today's world it's not necessarily about skills or talent.  Everyone has talent, and some are indeed better than others, but a lot of the content produced out there is not talent-driven, it's motivation-driven.  You probably are a better mapper than some of those guys already working in a studio, but the difference here is, they have much more motivation than you; after all, they made it there, and you haven't.  Also too, it's not about having the resources, but being resourceful.  The challenge really is about how to find motivation, and being able to ask and seek that is the stepping stone in admitting the true qualities of a successful level designer.  Your work may not be the best, but that's ok, completing it is far more telling and meaningful to a possible employer, than something beautiful and unfinished.  No body wants to see something unfinished (unless you're on an editing site :D).  So let that drive your ambition.  Finishing something is ALL that matters.  -And finishing it within a reasonable amount of time, is the true talent you have to discover by experience alone.
With knowledge and skills like that, you can make whatever you want!
                                            
                        Blog: 
www.playingarchitecture.net
LinkedIn: 
Eric Lancon
Twitter:
@Riven202